There are multiple compression neuropathies of the upper extremity. Some neuropathies, like carpal tunnel, are quite common; others like posterior interosseous nerve (PIN) syndrome are not. Knowledge of anatomy and function of each nerve is essential to diagnose which nerve and compression site is involved correctly. The posterior interosseous nerve is a branch of the radial nerve, which comes off the posterior cord of the brachial plexus. The radial nerve, with nerve roots C5 to T1, travels down the arm and divides into superficial and deep branches in the proximal forearm. Normally the deep branch of the radial nerve dives into the posterior forearm through the heads of supinator to emerge as the posterior interosseous nerve. Anatomical variants include the deep radial nerve passing through the Arcade of Frohse to become the posterior interosseous nerve. This variant can increase susceptibility to impingement. The posterior interosseous nerve supplies motor innervation to the posterior forearm. The terminal branch of the posterior interosseous nerve travels distally into the floor of the 4th dorsal compartment of the wrist to innervate the dorsal wrist capsule.
Compression neuropathies of the radial nerve distal to the elbow include radial tunnel syndrome, posterior interosseous nerve syndrome, and Wartenberg syndrome. Each of these has distinct symptoms which can help with identifying the correct diagnosis. Posterior interosseous nerve syndrome is a compressive neuropathy of the posterior interosseous nerve which innervates the extensor compartment of the forearm. It usually has an insidious onset, often presenting with weakness in finger and thumb extension. However, there should be preservation in wrist extension due to the radial nerve innervated extensor carpi radialis longus. It is often self-limiting and resolves with conservative measures. However, symptoms that are refractory to nonoperative treatment may require surgical decompression.